Automotive transmissions often have hydraulically-actuated, selectively engagable torque-transmitting mechanisms, referred to as clutches. The clutches may be stationary-type clutches (i.e., brakes), or rotating-type clutches. Typically, interspaced sets of friction plates and reaction plates are placed in contact with one another when the clutch is engaged, to cause common rotation (in the case of the rotating-type clutch) or grounding (in the case of the stationary-type clutch) of components connected with the respective sets of plates.
Hydraulically-actuated clutches have inherent spin losses which reduce fuel economy. Spin losses are caused by fluid in the clutch increasing resistance to rotation of the components connected with the clutch plates.
Additionally, hydraulically-actuated clutches are not engagable until sufficient fluid pressure is available from a transmission pump. Typically, a main transmission pump is driven by an engine that is connected with the transmission or is driven by a rotatable transmission member, and so is indirectly driven by the engine. When the engine is off, such as in some hybrid operating modes, or when the engine output member is rotating at relatively low speeds, the main pump may not provide sufficient apply pressure. Accordingly, auxiliary pumps, such as electrically-powered pumps, are sometimes provided, especially in a hybrid transmission, to provide hydraulic pressure when pressure from the main pump is insufficient.